(1) Technical Field
The present disclosure relates to energy band structure of organic materials in organic electroluminescence (EL) elements.
(2) Description of Related Art
In recent years, use of organic EL elements in display devices has become popular.
An organic EL element has a structure in which at least a light-emitting layer is sandwiched between an anode and a cathode. Organic EL elements also typically have a hole transport layer for supplying holes to the light-emitting layer, between the anode and the light-emitting layer, and an electron transport layer for supplying electrons to the light-emitting layer, between the light-emitting layer and the cathode.
When a voltage is applied between the anode and the cathode, electrons are injected from the electron transport layer into the lowest unoccupied molecular orbital (LUMO) of the light-emitting layer and holes are injected from the hole transport layer into the highest occupied molecular orbital (HOMO) of the light-emitting layer.
Further, in the light-emitting layer, an electric field is generated by the voltage applied between the anode and the cathode. Electrons injected to the LUMO of the light-emitting layer are influenced by the electric field and move, within the light-emitting layer, towards the anode. On the other hand, holes injected to the HOMO of the light-emitting layer are influenced by the electric field and move, within the light-emitting layer, towards the cathode.
In this way, the electrons and holes that move in the light-emitting layer recombine to generate excitons. The excitons generate light when returning to a ground state from an excited state (JP 2004-514257).
Various factors affect the lifespan of an organic EL element. Such factors include deterioration of organic material that forms the light-emitting layer. Deterioration of organic material is known to be facilitated by light emission.
Further, it is known that distribution of light emission intensity in a thickness direction of the light-emitting layer can have a sharp peak (hereinafter, unless specified otherwise, “distribution” indicates distribution in the thickness direction). A sharp peak in the distribution of light emission intensity means that degradation of the organic material is particularly facilitated at a position indicated by the peak.
Such localized degradation of the organic material is thought to limit the lifespan of the light-emitting layer, and therefore a sharp peak in distribution of light emission intensity, i.e., a sharp peak in distribution of recombination, is a technical problem that is a factor inhibiting increasing lifespan of organic EL elements.